The UN secretary-general, Ban Ki-moon, has called for a comprehensive review of the policy on biofuels as a crisis in global food prices - partly caused by the increasing use of crops for energy generation - threatens to trigger global instability.

"We need to be concerned about the possibility of taking land or replacing arable land because of these biofuels," Ban told the Guardian in Bucharest while attending this week's Nato summit. But he added: "While I am very much conscious and aware of these problems, at the same time you need to constantly look at having creative sources of energy, including biofuels. Therefore, at this time, just criticising biofuel may not be a good solution. I would urge we need to address these issues in a comprehensive manner."

Climate change has been a priority for Ban since he took over from Kofi Annan, and he has embraced the potential of biofuels, derived from plants, as a long-term substitute for fossil fuels. But as food prices have soared - driven by rising demand, high fuel costs, and climate change - the cultivation of biofuels has come under fire for diverting fertile land from food production.

Some of the loudest criticism has come from within UN food agencies, which are struggling to keep up with commodity prices. Last month the World Food Programme issued an emergency $500m appeal to donors to help it meet its existing commitments to the world's hungry.

WFP officials say 33 countries in Asia and Africa face political instability as the urban poor struggle to feed their families.

There are also mounting concerns over the benefits of biofuels to the environment. They generally burn cleaner than fossil fuels, but fuels such as grain-based ethanol are energy-intensive to produce, and tropical rainforests have been cleared to produce palm oil for use as a fuel.

The role of biofuels is under review in Britain pending an inquiry into the indirect impact of their cultivation by the Renewable Fuels Agency.

Against this backdrop some senior UN officials are pushing for a change of policy, and attack Ban in private. "Ban is just out of touch," one said. "He doesn't know what is really going on in our agencies."

The UN's own special rapporteur on the right to food, Jean Ziegler, called biofuels "a crime against humanity", and called for a five-year moratorium.

Ban rejected that proposal. "At this time I wouldn't make any definitive judgment or definitive plans, in particular vis-à-vis these biofuels," he said. "I know there are some concerns raised by certain quarters about biofuels. But biofuels are a renewable source of energy when we are experiencing extreme difficulties [with] resources."

But Ban conceded that there was a food supply problem and said the primary Millenium Development Goal of halving global hunger by 2015 looked harder to reach than ever. "This steeply rising food price is a new phenomenon," he said. "We have only seven years left to meet the target of 2015. This is very serious."

He said he was overseeing a multi-agency investigation of the issue involving the UN Energy Programme, the UN Development Programme, the Department of Economic and Social Affairs and the World Food Programme. "They are all working on this issue," the secretary-general said.